1. Field
Embodiments of the invention relate to designs of, and methods of using, an electro-optical phase modulator for use in an optical coherence tomography system.
2. Background
Optical Coherence Tomography (OCT) is a medical imaging technique providing depth resolved information with high axial resolution by means of a broadband light source and an interferometric detection system. It has found plenty of applications, ranging from ophthalmology and cardiology to gynecology and in-vitro high-resolution studies of biological tissues.
A common element found in both OCT systems and optical communication networks is a phase modulator designed to shift the phase and/or frequency of a beam of light. In a basic phase modulation (PM) scheme, the information of the modulating signal is transferred into the optical signal in the form of instantaneous phase variations, which translates into instantaneous frequency modulation. PM has been exploited in all-optical circuits finding application in various other fields, including signal processing, sensing and biophotonics.
Several technologies have been demonstrated as suitable for implementing PM of light. Among them, those implemented by Photonic Integrated Circuits (PICs) are useful since complete manufacturing on a single substrate using techniques similar to those found in microelectronics is feasible. As a result, final devices featuring a small footprint, high stability and reliability, and reduced manufacturing costs can be achieved. PM on PICs has been traditionally implemented by exploiting the Pockels effect in lithium niobate (LiNbO3). However, other technologies based on carrier recombination in indium phosphide (InP), silicon-on-insulator (SOI), liquid crystals (LC), polymers and the hybridization of materials such as III-V group materials on silicon or silicon-polymer mixtures have emerged in the past few years.
There exist two different physical mechanisms governing refractive index modulation in SOI structures. The first mechanism is the thermo-optic (TO) effect. The refractive index in silicon has a dependence on the temperature that stems from the lattice constant's dependence on the temperature. This effect translates into changes in the optical properties of the material thus modifying its refractive index. In particular, the TO coefficient in SOI is defined as dn/dT=2.4·10−4 K−1, with n being the refractive index and T being the temperature. The amplitude variation of the optical signal propagating through the SOI structure is negligible when modifying the temperature since it has a reduced influence on the imaginary part of the refractive index. This implies that no extra absorption is derived from the modulation process. However, the TO dynamics are typically limited to the scale of several tens to hundreds of microseconds, resulting in maximum operating rates in the order of tens to hundreds of kilohertz. Such operating rates may not be fast enough for certain imaging techniques based on OCT.
The second mechanism governing refractive index modulation in SOI structures is known as the plasma dispersion (PD) effect. The PD effect is based on the injection or depletion of free carriers into the intrinsic silicon comprising the SOI waveguide structure. The free carrier concentration has strong influence on both the imaginary and real parts of the refractive index of the material. The efficiency of the refractive index change for the PD effect is higher compared with the TO effect, since larger modifications can be reached in the real part of the refractive index. However, the mentioned variations in the real part are accompanied by a change in its imaginary part as well, resulting in extra light absorption owing to the carrier density modulation. Consequently, due to the extra light absorption, the PD effect in silicon induces a Residual Amplitude Modulation (RAM) to the modified signal. A solution consisting of filtering out the output modulated signal has been proposed in U.S. Pat. No. 7,167,293 for the purpose of reducing the RAM in PD-based SOI optical modulators. However, to completely mitigate the RAM, perfect topology symmetry and fine tuning of the filters are required. As a result, the perfect mitigation of the RAM becomes challenging and its effect can still be high enough to drown out small signal variations carrying important information for some applications.